1. Field of the Invention
The present invention relates to a control device for a Hall element motor, particularly to a control circuit which can obtain a constant speed rotation by controlling the input voltage of a Hall element by the comparative output between a voltage induced at each field winding when the motor is rotated and a constant voltage of reference speed.
2. Description of the Prior Art
In a Hall motor in which the position of a rotor consisting of a permanent magnet is detected by a Hall element for changing over the driving current flowing to each field winding, a method has been effected which detects the voltage induced at each field winding so that the input voltage of each Hall element is controlled thereby. Heretofore, such voltage as corresponding to the rotation speed of a rotor is induced at the above mentioned field winding, and such induction voltage which corresponds to the rotation speed of the rotor, that is, the rotation speed of a motor is detected by a comparator circuit consisting of a switching transistor circuit. Then the switching transistor circuit is reversed when the above mentioned induction voltage reaches a prescribed level or higher for controlling the input current to the hall element thus controlling the driving current flowing to the field winding, thereby a prescribed torque is given to a motor for providing always a constant speed to a motor. In such conventional control circuit, the above mentioned induction voltage is impressed between a base and an emitter of a switching transistor when the rotation speed of a motor is detected by a comparator circuit so that the V.sub.BE of the switching transistor is a voltage which corresponds to the reference rotation speed for making comparison. However, as the induction voltage is generated in a form which is overlapped with power source voltage, when the power source voltage varies by some reason, even if the induction voltage is kept constant, the voltage impressed to the base-emitter of the switching transistor varies. Therefore, not only is there a difficulty that correct comparison can not be made, but the V.sub.BE of the transistor itself varies against the change in temperature thus correct speed control can not be effected.